A fuel cell is a device that generates electricity by a chemical reaction. Among various fuel cells, solid oxide fuel cells use a hard, ceramic compound metal (e.g., calcium or zirconium) oxide as an electrolyte. Typically, in solid oxide fuel cells, an oxygen gas, such as O2, is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as H2 gas, is oxidized with the oxygen ions to form water at the anode. Fuel cells are generally designed as stacks, whereby subassemblies, each including a cathode, an anode and a solid electrolyte between the cathode and the anode, are assembled in series by locating an electrical interconnect between the cathode of one subassembly and the anode of another.
A major materials challenge in solid oxide fuel cell (SOFC) development is the interconnect material, which provides the conductive path for electrical current to pass between the electrodes and to the external circuit. The interconnect also acts as a separator plate, physically separating the fuel in the anode cavity from the air or oxygen in the cathode cavity. Therefore, the interconnect material must have good electrical conductivity to minimize resistive losses and must be stable in both oxidizing and reducing conditions. SOFCs operate at high temperatures, and therefore the coefficient of thermal expansion (CTE) of the interconnect material must be close to those of the other cell components in order to minimize thermal stresses. Other requirements of interconnect materials include adequate mechanical strength, relatively low permeability to oxygen and hydrogen, and reasonable thermal conductivity. Jeffrey W. Fergus, Lanthanum chromite-based materials for solid oxide fuel cell interconnects, Solid State Ionics, 171 pp. 1-15 (2004); Zhenguo Yang, Recent advances in metallic interconnects for solid oxide fuel cells, International Materials Reviews, 53[1] pp. 39-54 (2008).
There are two approaches, in general, to developing interconnect materials for SOFCs: metallic and ceramic, each of which offers different advantages and challenges. Metallic interconnects have good electrical conductivity, but are not stable when exposed to the oxidizing conditions at the cathode of SOFCs, so that coating of the surface with a conductive oxide (such as spinels) is usually required. Ceramic interconnect materials are oxides, and are thus stable in oxidizing atmospheres, but typically have lower electrical conductivities as compared to metals. High cost of the raw materials and fabrication difficulties in obtaining high density are some other issues related to ceramic interconnects which have hindered their application in SOFCs.
Lanthanum-doped strontium titanates (LST) are promising interconnect materials with good chemical and dimensional stability in reducing environments, low oxygen ion mobility and acceptably low resistance when used as thin layers. They also exhibit good compatibility with both cathode and anode materials without formation of deleterious second phases. However, sinterability typically is a limitation when co-firing with yttria-stabilized zirconia (8 mol % YSZ) electrolytes. Currently, LST materials generally need to be sintered at 1,460-1,500° C. to enable complete densification, while YSZ (such as, for example, TZ-8Y from Tosoh (Tosoh USA, Grove City, Ohio)) can be sintered to full density at 1,350° C. Therefore, LST materials generally need to be modified either physically or chemically to improve their sinterability to enable co-firing with YSZ electrolyte.